Lean air-fuel mixture combustion technique for performing the spark ignition of the lean air-fuel mixture (of which the air-fuel ratio is large) is well known, for example, as shown in Patent document 1 (shown below). In this lean air-fuel mixture combustion technique, NOx contained in the exhaust gases cannot be removed by the three-way catalyst. Accordingly, it is important to make the combustion limit air-fuel ratio become higher. Patent document 1 discloses a technique for stably burning the mixture of which the average air-fuel ratio in the combustion chamber is high, by stratifying the mixture so that a portion of the mixture having a relatively rich air-fuel ratio (small air-fuel ratio) exists in the vicinity of the spark plug.
However, according to this technique, a comparatively large amount of NOx is generated by the combustion in the vicinity of the spark plug. It is therefore desired to further reduce an amount of NOx emission.
In view of the above, the inventors of the present invention proposed spark ignition combustion of a homogeneous lean air-fuel mixture wherein the homogeneous lean air-fuel mixture is burned by spark ignition, thereby ensuring stable combustion as well as reducing an amount of NOx emission (Non-patent document 1). In this non-patent document 1, as a method for surely burning the homogeneous lean mixture of an air-fuel ratio around “30”, setting the compression ratio to such a value that a temperature of the air-fuel mixture at the ignition timing is raised to the desired temperature, using a fuel injection valve capable of injecting atomized fuel, and the like are proposed.